#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (ll)(n); i++)
#define all(a) (a).begin(), (a).end()
using ll = long long;
const int INF32 = 2e9;
const ll INF64 = 4e18;

int main() {
    // 入力.
    ll N, M, P, Y;
    cin >> N >> M >> P >> Y;
    vector<ll> A(N), B(N), C(N), E(N, INF64);
    vector<vector<pair<ll, ll>>> G(N);
    for (int i = 0; i < M; i++) {
        cin >> A[i] >> B[i] >> C[i];
        A[i]--;
        B[i]--; // 0-indexed.
        G[A[i]].push_back(make_pair(B[i], C[i]));
        G[B[i]].push_back(make_pair(A[i], C[i]));
    }
    for (int i = 0; i < P; i++) {
        ll Di, EDi;
        cin >> Di >> EDi;
        Di--;// 0-indexed.
        E[Di] = EDi;
    }

    // ダイクストラ法に必要な配列等.
    vector<ll> katsuage(N, INF64);
    vector<bool> confirmed(N, false);
    priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>>
        PQ;

    // 始点.
    katsuage[0] = 0;
    PQ.push(make_pair(katsuage[0], ll(0)));

    // ダイクストラ法.
    while (!PQ.empty()) {
        int pos = PQ.top().second;
        PQ.pop();

        if (confirmed[pos])
            continue;

        confirmed[pos] = true;
        for (int i = 0; i < G[pos].size(); i++) {
            int next = G[pos][i].first;
            int cost = G[pos][i].second;
            if (katsuage[next] > katsuage[pos] + cost) {
                katsuage[next] = katsuage[pos] + cost;
                PQ.push(make_pair(katsuage[next], ll(next)));
            }
        }
    }

    // ハチマキが何本買えるか.
    ll ans = 0;
    for (int i = 0; i < N; i++) {
        ans = max(ans, (Y-katsuage[i])/E[i]);
    }

    cout << ans << endl;

    return 0;
}